he probably doesnt appreciate people pretending to be him.uchiha ego.besides the ten tails would change a lot he'll push tobi to his limit.
Tobi + Madara = Two masterminds who work together = They lead the project Tsuki No Me.
i think i'd be funny Madara would be like "YO i hear there is a guy who says they are me?" and Tobi "Uh..." *walks away slowly* "Oh i think the Moon eye plan is calling to me gtg" *Flees*